I fitted an AIS receiver last year, the Garmin with built in VHF radio. I really didn't get on with the radio so decide to replace it. There is a Garmin receiver with out vhf but I found AIS such a useful tool I decided to replace with a Transmitter/receiver (Garmin AIS 600). The only drawback to AIS currently is that only a small percentage of boats have transmitters, I couldn't moan about it if I didn't have one myself so I joined the club.
The Garmins have a proximity alarm for AIS targets, it's a PITA when in/near harbours as it goes off constantly, can be turned off easily but it switches on every time you turn the plotter on. But it is a safety feature, at least you don't need to worry about manually turning it on. The parameters are adjustable too.
It goes without saying, if everyone went out and bought a receiver without t/x then we'd all be wasting our time and money, there'd be nothing to see.
Obviously it's a legal requirement for anything over 200 grt, personally I hope it goes the same way as DSC and a transmitter becomes law for everyone.
If you can afford it, take the initiative and buy one!
. Having said all that, it's only as good as me operating the plotter, and obviously the same applies to the person watching the screen on any boat heading my way (if I had a transmitter too). There is a chance a coastguard might spot impending danger and warn one or other vessel if they think there is a collision imminent. I have been toying with the idea of a transmitter too, but take the view that if it is foggy, I will be glued to my screen watching for AIS targets anyway. Never rely on the other vessel seeing you is my motto.
